1. Field of the Invention
The present invention relates in general to collision warning systems and more particularly to collision warning systems of a type which is mounted on a motor vehicle issue an alarm when the vehicle approaches near an obstacle (viz., front vehicle) running in front of the motor vehicle.
2. Description of the Prior Art
In order to clarify the task of the present invention, one collision warning system of the above-mentioned type will be described with reference to FIGS. 5, 6 and 7 of the accompanying drawings.
For ease of understanding, in the following description, the vehicle on which the collision warning system is mounted will be referred to as a rear vehicle, and an obstacle (viz., another vehicle) which is running in front of the rear vehicle will be referred to as a front vehicle.
In FIG. 5, denoted by numeral 1 is a distance detection part which, by using laser beams emitted from laser diodes (which will be referred to as "LD" for ease of description), detects the distance (which will be referred to as "vehicle-to-obstacle distance" hereinafter) between the rear vehicle and the obstacle (viz., the front vehicle) in front of the rear vehicle. Denoted by numeral 2 is a running condition detection part which detects the running condition of the rear vehicle.
Denoted by numeral 3 is a signal processing part which processes information signals issued from the distance detection part 1 and the running condition detection part 2. That is, upon receiving an instruction signal from the signal processing part 3, the distance detection part 1 issues information signals which represent the vehicle-to-obstacle distance. By analyzing the information signals of the vehicle-to-obstacle distance and information signals representing the vehicle running condition which are issued from the running condition detecting part 2, the signal processing part 3 derives a relative speed between the rear vehicle and the obstacle for judging whether the obstacle is stationary or moving. Furthermore, by analyzing the vehicle speed, the relative speed, a personal equation of the driver in braking the vehicle and so on, the signal processing part 3 judges whether or not the rear vehicle has a possibility of collision against the obstacle.
Designated by numeral 4 is a display and alarm issuing part which, based on the processed data from the signal processing part 3, displays the vehicle-to-obstacle distance and issues an audible and/or visual alarm when the possibility of vehicle collision is determined by the signal processing part 3.
The running condition detection part 2 includes a vehicle speed sensor 31. The signal processing part 3 includes an arithmetic circuit 41 and a distance determining switch 42. Information signals issued by the vehicle speed sensor 31 are fed to the arithmetic circuit 41, and when a vehicle speed derived by analyzing the signals from the vehicle speed sensor 31 exceeds 35 km/h, the arithmetic circuit 41 issues an instruction signal to the distance detection part 1.
The instruction signal from the arithmetic circuit 41 is received by a drive signal generating circuit 11 of the distance detection part 1. Upon receiving the instruction signal, the drive signal generating circuit 11 feeds an LD switch driver 12 with an LD energizing signal which is shown in FIG. 6(a) Upon receiving the energizing signal, the LD switch driver 12 issues three types of signals which are shown in FIGS. 6(b), 6(c) and 6(d). With this, three laser diodes LD-L, LD-C and LD-R which are light emitting means of an LD array 13 are energized to emit light (viz., laser beam) having a constant intensity one after another.
The laser beams generated by these three laser diodes LD-L, LD-C and LD-R are projected through a common lens 14 forward but slightly leftward, centrally forward and forward but slightly rightward from the vehicle, respectively. Thus, the laser beam from the laser diode LD-C can be used for detecting the front vehicle which is running in front of the rear vehicle, while, the laser beam from the laser diode LD-L or LD-R can be used for detecting another front vehicle which is running into the lane of the rear vehicle from a left or right side.
The laser beam reflected by the front vehicle is collected by a collecting lens 15 and received by photo diodes 16 (PD). Upon receiving the reflected laser beam, the photo diodes 16 issue corresponding signal to an amplifying circuit 17. By amplifying the signal, the amplifying circuit 17 issues a signal as shown in FIG. 6(e). However, practically, the signal received by the amplifying circuit 17 includes object signals B reflected by the front vehicle and noise signals A reflected by adjacent road surfaces.
Designated by numeral 18 is a threshold determining circuit which, upon receiving the LD energizing signal (viz., the signal shown by FIG. 6(a)) outputted by the drive signal generating circuit 11, generates a threshold signal, as shown in FIG. 6(f), which is fed to a comparator 19. By comparing the level of the signal (viz., the signal shown by FIG. 6(e)) from the amplifying circuit 17 with that of the threshold signal of FIG. 6(f), the comparator outputs only a signal based on the object signal reflected by the front vehicle, which signal is shown in FIG. 6(g).
Designated by numeral 20 is a counter which, upon receiving the LD energizing signal (FIG. 6(a)), starts counting of clock pulses fed by a reference pulse generating circuit 21, and, upon receiving the pulse signal of FIG. 6(g), stops the counting. From the time counted up by the counter 20, an information signal representing the distance between the vehicle and the front vehicle is derived and the signal is fed to the arithmetic circuit 41.
Receiving various information signals from the counter 20 and the vehicle speed sensor 31, the arithmetic circuit 41 carries out a judgement as to whether or not the rear vehicle has a possibility of collision against the front vehicle.
In the arithmetic circuit 41, programmed operation steps shown in the flowchart of FIG. 7 are carried out.
That is, upon energization of the collision warning system, the operation steps start. Then at step 11, the information signal representing a vehicle-to-obstacle distance "R" between the rear vehicle and the front vehicle and the information signal representing the running speed "V.sub.f " of the rear vehicle are read. This reading is carried out every given period.
At step 12, the signal representing the vehicle-to-obstacle distance "R" is converted to a display signal which is fed to a distance display device 51 (see FIG. 5). This device 51 displays the distance "R" thereon. Then at step 13, a relative speed [(d/dt)R] between the rear vehicle and the front vehicle and the running speed (V.sub.a) of the front vehicle are derived. That is, the relative speed is provided by differentiating the vehicle-to-obstacle distance "R" with an aid of the method of least square or the like. Furthermore, the speed (V.sub.a) of the front vehicle is provided by treating the speed (V.sub.f) and the relative speed [(d/dt)R].
When the value [(d/dt)R] is smaller than 0 (zero), it is judged that the vehicle-to-obstacle distance "R" is reducing, and, when the value [(d/dt)R] is greater than 0 (zero), it is judged that the vehicle-to-obstacle distance "R" is increasing. While, when the value [(d/dt)R] is 0(zero), it is judged that the vehicle-to-obstacle distance "R" is kept unchanged.
In order to carry out the judgement as to whether the rear vehicle has a possibility of collision against the front vehicle, the following method is employed.
That is, in case wherein the speed of the rear vehicle is represented by "V.sub.f "(m/s), the speed of the front vehicle is represented by "V.sub.a "(m/s), the braking performance of the rear vehicle is represented by ".alpha." (m/s.sup.2), and the time (which is set by the distance determining switch 42) elapsed from the time when the driver notices a danger (viz., alarm) to the time when he or she practically actuates the brake pedal is represented by "Td", the following equation is provided for deriving a so-called critical vehicle-to-obstacle distance "Rc" which is used for judging possibility of the collision. EQU Rc=V.sub.f .times.T.sub.d +(V.sub.f.sup.2 -V.sub.a.sup.2)/2.alpha.(1)
wherein: PA1 wherein: PA1 wherein: PA1 wherein:
V.sub.f .times.T.sub.d : distance traveled by the rear vehicle during the time "Td", PA2 V.sub.f.sup.2 /2.alpha.: stopping distance of rear vehicle, and PA2 V.sub.a.sup.2 /2.alpha.: stopping distance of front vehicle. PA2 T.sub.d : fixed time period, PA2 .alpha.: braking performance of the vehicle; PA2 .alpha.: braking performance of the vehicle; PA2 .alpha.: braking performance of the vehicle;
Then, at step 14, a comparison between the relative speed (d/dt)R and the speed V.sub.f of the rear vehicle is carried out. If, at this step, an equation "-(d/dt)R=V.sub.f " is established, that is, when the front vehicle is kept stopping (viz., V.sub.a =0), the operation flow goes to step 15. At this step, by using the following equation, a judgement is carried out for determining the possibility of collision against the stopping front vehicle. EQU V.sub.f .times.T.sub.d +V.sub.f.sup.2 /2.alpha..gtoreq.R (2)
If YES at step 15, that is, if the measured distance between the rear and front vehicles is equal to or smaller than the value "V.sub.f .times.T.sub.d +Vf.sup.2 /2.alpha.", the operation flow goes to step 18 judging actualization of possibility of the vehicle collision. At this step 18, an instruction signal is fed to an alarm issuing device 52 (see FIG. 5) to energize the same for letting the driver know the danger. Upon noticing the alarm, the driver can actuate the brake pedal for reducing the speed of the vehicle or stopping the same.
While, if NO at step 14, that is, if the equation "-(d/dt)R=V.sub.f " is not established, that is, if the front vehicle is running, the operation flow goes to step 16. At this step, a judgement is carried out as to whether the relative speed [(d/dt)R] is equal to or greater than a predetermined speed "C" (m/s) or not.
If YES at step 16, that is, if the equation "(d/dt)R.gtoreq.C" is established due to a higher relative speed, the operation flow goes to step 15 considering that the rear vehicle is rapidly getting near the front vehicle. That is, in this condition, due to the higher relative speed, the speed of the front vehicle is considered to 0 (zero). Thus, thereafter, the judgement as to issuance of the emergency alarm depends on the equation (2), which has been described hereinafore.
If NO at step 16, that is, if the equation "(d/dt)R.gtoreq.C" is not established due to a lower relative speed, the operation flow goes to step 17 considering that the rear vehicle is following the front vehicle keeping a constant distance therebetween. That is, in this condition, the speed "V.sub.a " of the front vehicle is considered substantially equal to the speed "V.sub.f " of the rear vehicle. Thus, the equation (1) can be converted to the following equation. EQU Rc=V.sub.f .times.T.sub.d ( 3)
At step 17, the judgement of possibility of collision is carried out by using the following equation (4). EQU V.sub.f .times.T.sub.d .gtoreq.R (4)
That is, if YES, that is, if the measured distance between the rear and front vehicles is equal to or smaller than the value "V.sub.f .times.T.sub.d ", the operation flow goes to step 18 to issue the emergency alarm.
However, due to its inherent construction, the above-described conventional collision warning system has the following drawbacks.
That is, usage of the equation (1) (or (2)) as means for judging possibility of vehicle collision induces unsuited alarming operation when the vehicle (rear vehicle) is moving at high speed on an expressway or the like. In fact, in a case wherein both the front and rear vehicles are running at high speed, the value "(V.sub.f.sup.2 -V.sub.a.sup.2)/2.alpha." of the equation (1) largely affects the value "Rc" even if the speed difference (V.sub.f -V.sub.a) between the rear and front vehicles is small, which causes an extremely large value of the critical vehicle-to-obstacle distance "Rc". Thus, in high speed cruising of the vehicle, the emergency alarm fails to be issued in a desired manner.
Furthermore, usage of the equation (1) (or (2)) for judging the possibility of vehicle collision tends to bring about an-unnecessary alarm. In fact, such emergency alarm is sometimes issued even when a sufficiently long distance is left between the rear vehicle and the front vehicle. That is, because the time "T.sub.d " is a fixed value set by the distance determining switch 42, the value "V.sub.f .times.T.sub.d " is proportionally increased with increase of the speed "V.sub.f " of the rear vehicle, which increases the critical vehicle-to-obstacle distance "Rc". Thus, even when the driver is just ready for effectively braking the vehicle with his or her foot kept on the brake pedal under cruising of the vehicle, an alarm tends to be issued irrespective of a sufficiently long distance kept between the front and rear vehicles.